The present invention utilizes a novel adaptive inverse continuously variable delta slope (ICVSD) modulation and demodulation technique to provide for a method and apparatus for high-speed data communications. This technique provides a far greater degree of performance and lower production costs, when compared to prior art modem methods and apparatus (i.e., based on dedicated hardware or advanced DSP-based.)
With a view towards a practical application such as enabling mobile or remote data communications via an RF communications path, a brief overview of the adaptive inverse continuously variable delta slope (ICVSD) modulation and demodulation technique and its relevant background will now be given. A typical RF communications path is established electromagnetically between two physically separate points via the action of an RF transmitter and an RF receiver. Typically a fixed radio frequency is utilized between said two points, although spread spectrum RF methods are currently finding more and more favor (although even in spread spectrum, one dwells on a fixed frequency for at least a minimum time duration necessary to convey the minimum information quantity necessary or desired).
At the present time, said RF frequency (i.e., carrier) is typically frequency modulated (FM). This is the general choice in analog systems, the majority of these being trunked systems in use for voice communications within mobile systems. One advantage of FM is it's high signal to noise ratio within a given bandwidth. Amplitude modulation of the RF carrier is also found to be used, especially at lower radio frequencies, and new techniques for phase modulation, or more recently, direct digital generation and modulation of an RF carrier, are also used. By far the most dominant though, is said analog FM modulation for providing voice communications.
In an effort to convey discrete data over such an FM-based analog voice bandwidth communication channel or path, various baseband signaling and modulation schemes have been utilized. These schemes range from FSK through PSK, QPSK, QAM, GMSK etc., with attendant higher data rates and increasing modem complexity. Practical implementations of the above schemes have not been able to provide data rates anywhere near the theoretical information capacity of a communications channel, because aside from the theoretical information capacity limit determinants (i.e., channel bandwidth and signal to noise ratio), a practical RF communications path represents a baseband analog channel that suffers from varying amounts of phase shift and DC offsets. This is because the RF transmitters modulator and RF receivers discriminator (demodulator) usually have their baseband signals AC coupled.
As such, only schemes that are relatively tolerant of DC level offsets etc. may be employed, and these are all of low spectral efficiency (i.e., ratio of discrete data to occupied baseband spectrum.) Accordingly, in an effort to utilize the more complex schemes (which were originally developed for PSTN data communications), those affording higher data rates and increased spectral efficiency (as well as increasing sensitivity to said effects and of ever increasing complexity), some RF transmitters and RF receivers have been modified or designed to provide DC coupling to and from said modulator and demodulator respectively. With A well designed transmitter and receiver, this should allow greatly increased data rate throughput for a given analog baseband bandwidth and signal to noise ratio, and it does. Excepting the fact that various drift effects between said transmitter and receiver inevitably lead to DC level shifts and offsets. This once again has imposed an upper bound or limit on practically achievable data rates with said prior art.
In the novel method according to one embodiment of the present invention, the technique employed presumes the de facto occurrence and existence of DC level shifts and offsets, as welt as phase shifts. The apparatus according to the present invention, continually adapts itself to these normally slowly changing shifts and offsets.
Although generically speaking, the method and apparatus according to the present invention falls under the heading of modulation and demodulation known as the continuously variable slope delta type, which is well known in the art, its use in an inverse form specifically for the conveyance of discrete data over an analog baseband channel, as in the present invention, appears to be unknown and unanticipated in and by the prior art.
Continuously Variable Slope Delta (CVSD) modulation (and demodulation) is normally associated with digital speech encoding and decoding, wherein it Is applied specifically for the digital communication of analog signals. As such, CVSD has been extensively used for the conversion of analog signals into a serial bit stream (discrete data) for subsequent digital transmission.
The key component in a CVSD system is the delta modulator which is well understood in the art. A delta modulator consists at a comparator in the forward path and an integrator in the feedback path of a simple control loop, The comparator has as its inputs a bandlimited analog input signal an the output or the integrator, The comparator output reflects the sign of the difference between the analog input signal magnitude and the integrator output amplitude at any instance in time. This sign bit is the actual digital output and also control the direction of ramp of the integrator. The comparator may be suitably clocked, its output thereby producing a synchronous and bandlimited digital bit stream.
If in such a CVSD system, the clocked serial bit stream from the above modulator is transmitted, received and applied to a similar integrator at the remote receive point, the receive integrator output is a copy of the transmitting control loop integrator output. In this manner, one may say that the transmitting integrator is tracking the original analog input signal and as a consequence, that the receive integrator is reproducing the said input signal being tracked. Appropriate low pass filtering of the receive Integrator output will remove most of the quantization noise, if the clock rate of the bit stream is an octave or more above the bandwidth of the input signal (i.e., the Nyquist rate.) In the case of voice communications having a bandwidth of 4 kHz, clock rates of 8 kHz and up would be used.
It can thus be seen that the delta modulator digitizes and conveys the analog input signal to a remote delta demodulator. The simplicity of the approach and the serial unframed nature of the encoded data stream has been ideal for application in data communications networks. Further, with a loss of input signal to the transmitting delta modulator, a continuous one-zero alternation is generated and transmitted. If the two integrators are made leaky, then during any loss of contact (i.e., disruption of the digital bit stream) the receiving delta demodulator output decays towards zero and receive restart can occur without any framing considerations when the receiving delta demodulator reacquires its digital input. Also, it should be noted that a delta demodulator is quite tolerant of sporadic bit errors.
The basic delta modulator/demodulator function described so far does not include the continuously variable slope aspect that is key to the improved performance of CVSD. Accordingly, with the basic delta modulator there are limitations with regard to the ability to accurately convert the input within a limited digital bit rate. The analog input signal must be bandlimited and amplitude limited. The frequency limitations are governed by the Nyquist rate, while the amplitude capabilities are governed by the gain of the integrator.
The frequency limits are bounded on the upper end, that is, for any input bandwidth there exists a clock frequency larger than that bandwidth required to transmit the signal with a specific noise level. However, the amplitude limits are bounded on both the upper and lower ends. Hence, for a given signal level, one specific gain will achieve an optimum noise level. Unfortunately, the basic delta modulator has a small dynamic range over which the noise level is optimally constant
The addition of the continuouely variable slope (CVS) function to the basic delta modulator provides increased dynamic range by continuously adjusting the gain of the integrator. For a given clock frequency and input bandwidth, the CVS function therefore serves to increase the dynamic range of the delta modulator. This is done by adding an algoithm to the basic delta modulator which monitors the past few outputs of the delta modulator in a simple shift register.
This shift register is most often 3 to 4 bits in length and the accepted CVSD algorithm simply monitors the contents of the shift register and indicates if it contains all one's or zero's. This condition is called coincidence, and when it occurs, it indicates that the gain of the integrator is too small. Such a coincidence outputs is used to charge a low pass filter (called a syllabic filter in voice coding applications), whose voltage output is used to control the integrator gain through a pulse width modulator whose other input is the sign bit (i.e., up/down control). This form of algorithm provides a measure of the average power or level of the analog input signal.
This algorithm is repeated in the receiver and level data is thereby recovered. Because the algorithm only operates on the past serial data, it effectively changes the nature of the bit stream without changing the channel bit rate. The overall effect of the CVS algorithm is to compand the analog input signal.
Due to the simplicity, robustness and spectral efficiency of CVSD when applied towards digital voice communications, it was theoretically, mathematically and experimentally determined that one might utilize a novel and very special form or arrangement of such said CVSD modulator and demodulator, to obtain a function not yet known in the art.
In the present invention, at the transmit end, a CVSD demodulator is utilized in an inverse or “mirror” fashion (when compared to its prior art use for the analog decoding of a digital bit stream originating from a CVSD modulator), in order to provide for a modulation scheme that generates a bandlimited analog output signal in response to a synchronous and bandlimited digital bit stream (i.e., serial bit stream) presented at its digital input. Said bandlimited analog output signal (baseband) can then be transmitted (via media suitable for analog voice communication, e.g., radio) to a remote receive print.
At said remote receive point, a CVSD modulator is also utilized in an inverse or “mirror” fashion (compared to its prior art use for the digital encoding of analog signals originating from a source of analog intelligence), in order to provide for a modulation scheme that generates a synchronous and bandlimited digital output bit stream (i.e. serial bit stream) in response to a bandlimited analog input signal presented to its analog input.
The above novel approach to the analog transmission of digital data is especially attractive for use in AC coupled RF communication paths, wherein DC level shifts and offsets, as well as phase shifts have a large occurrence likelihood. Aside from the application towards fixed or mobile RF data communications, the present invention also has application and promise for use in more physical communication paths or channels, such as the public switched telephone network, fiber optics, etc.